IColumn and related code removal #1755
Description
Years ago I wrote this code.
https://github.com/dotnet/machinelearning/blob/master/src/Microsoft.ML.Data/Data/IColumn.cs
That was fine, but many of the conveniences that this offered were not heavily used throughout the codebase, and the few places where it was convenient have been supplanted, I think, by the new Schema.Column, Schema.Metadata, and MetadataBuilder idioms.
This is work done as a prelude to #1532, since that change would heavily affect this code here, and this code serves a limited purpose, we may as well remove it.